Abstrict The present invention describes a method and apparatus for conditioning
air utilizing a desiccant based air conditioning system requiring
substantially less regeneration energy than typical systems. This
regeneration energy reduction is accomplished through the use of
two separate desiccant devices and an indirect evaporative cooler
having both a wet and dry side for air flow-through. The first desiccant
device regeneration air is first passed through the wet side of
the indirect evaporative cooler wherein it is humidified and heated.
This air is then dehumidified by passing through the second desiccant
device which operated at a high moisture content. This results in
a substantial amount of moisture being adsorbed from the first regeneration
air stream causing a substantial air temperature increase and thereby,
reducing the auxiliary heat required. The second desiccant device
may be regenerated with ambient air.
Claims In the claims:
1. An improved method of conditioning air
comprising the steps of dehumidifying the air to be conditioned
by contacting this air with a first air drying means,
then cooling the air to be conditioned by passing it through a
dry side of an indirect evaporative cooler,
then further cooling and humidifying the air to be conditioned
by passing it through a direct evaporative cooler, after which the
air is fully conditioned;
wherein the first drying means is regenerated by using a second
air stream comprising ambient air which is prepared for regeneration
by first warming and humidifying this second air stream by passing
it through a wet side of said indirect evaporative cooler, then
dehumidifying the second air stream by contacting it with a second
air drying means, then heating the second air stream, then passing
the second air stream into contact with the first air drying means
whereby the first air drying means is regenerated; and
wherein passing a third air stream comprising ambient air into
contact with the second air drying means to regenerate the second
air drying means.
2. The method of claim 1 wherein the first and second air drying
means are desiccants.
3. The method of claim 1 wherein the first and second air drying
means are separate components.
4. The method of claim 3 wherein the first and second air drying
means have differing moisture retention capacities.
5. The method of claim 4 wherein the moisture retention capacity
of the second air drying means is greater than the moisture retention
capacity of the first air drying means.
6. The method of claim 2 wherein the desiccants are selected from
a group comprising silica gel, activated alumina, molecular sieves,
and hygroscopic salts.
7. The method of claim 2 wherein the first and second desiccants
are each affixed to a rotatable desiccant wheel apparatus.
8. The method of claim 7 wherein the processes of adsorption and
desorption occur simultaneously on the same desiccant wheel apparatus.
9. An air conditioning system comprising
a first air drying means containing a dehumidification compartment
and a regeneration compartment each of which having an inlet and
outlet for air flow through,
an indirect evaporative cooling means with a wet and dry side each
of which having an inlet and outlet for air flow through,
a direct evaporative cooling means with an inlet and outlet for
air flow through, and
means to regenerate the first air drying means comprising a second
air drying means containing a dehumidification compartment having
an inlet and outlet for air flow through and a regeneration compartment
having an inlet and outlet for air flow through and comprising a
heating means having an inlet and outlet for air flow through;
wherein the first air drying means dehumidification compartment
outlet is connected to the indirect evaporative cooler dry side
inlet and the indirect evaporative cooler dry side outlet is connected
to the direct evaporative cooler inlet; and
wherein the indirect evaporative cooler wet side outlet is connected
to the second air drying means dehumidification compartment inlet,
the second air drying means dehumidification compartment outlet
is connected to the heater means inlet, and the heater means outlet
is connected to the first air drying means regeneration compartment
inlet; and wherein the second air drying means regeneration compartment
inlet is connected to a supply of ambient air.
10. The system of claim 9 whereby the first air drying means is
regenerated by an ambient air stream passing through the wet side
of the indirect evaporative cooler, and through the dehumidification
compartment of the second air drying means, and through the heater
means, and then through the regeneration compartment of said first
air drying means.
11. The system of claim 9 whereby the second air drying means is
regenerated by passing ambient air through the regeneration compartment
of said second air drying means.
12. The system of claim 9 wherein the first and second air drying
means are desiccants.
13. The system of claim 9 wherein the first and second air drying
means are separate components.
14. The system of claim 13 wherein the first and second air drying
means have differing moisture adsorbing properties.
15. The system of claim 14 wherein the second air drying means
has greater moisture adsorbing capabilities than the first air drying
means.
16. The system of claim 12 wherein the desiccants are selected
from a group comprising silica gel, activated alumina, molecular
sieves, and hygroscopic salts.
17. The system of claim 12 wherein the first and second desiccants
are each affixed to a rotatable desiccant wheel apparatus.
Description FIELD OF THE INVENTION
This invention relates to a method and apparatus for conditioning
air. More specifically, this invention conditions air through the
use of an improved desiccant based air conditioning system which
requires substantially less energy to regenerate the desiccant than
previously known systems.
BACKGROUND OF THE INVENTION
Desiccant based air conditioning systems have been finding increased
usage during recent years. These systems have been used to solve
certain HVAC problems that conventional vapor compression refrigeration
systems are ill-equipped to handle. For example, desiccant based
air conditioning systems have been used in applications where better
humidity control is required. This is due to the fact that desiccant
systems are capable of drying the air to a lower relative humidity
than conventional systems without frost development.
In addition, desiccant systems have been used where microbiological
growth is a concern. Desiccant systems do not require the "wet
surface" evaporator coil which is common to conventional systems.
This coil, along with its associated condensate collection basin,
can create a prime biological breeding ground. Also, tests have
shown that some desiccant systems can effectively remove bacteria
from the air stream with which the desiccant is brought in contact.
Desiccants can be solid, liquid, or gaseous substances which have
as a basic characteristic the ability to attract and hold relatively
large quantities of water. If, in attracting and holding moisture
the desiccant undergoes a chemical change, the process is called
absorption. If, in attracting and holding moisture the desiccant
undergoes a physical change only, the process is called adsorption.
In general, most absorbents are in liquid form and most adsorbents
are in solid form.
In many commercial air-conditioning applications where desiccants
are used, the desiccant is in solid form and adsorbs moisture from
the air to be conditioned. Examples of these types of desiccants
are silica gel, activated alumina, molecular sieves, or hygroscopic
salts. In some cases, these desiccants are contained in "beds"
over which the air to be conditioned is passed. Many times, however,
the desiccant is contained in what is known as a "Desiccant
Wheel".
A desiccant wheel is an apparatus typically comprising a plurality
of closely spaced, very thin sheets of plastic or metal which are
coated with a desiccant material. The wheel is contained in a duct
system that is divided into two sections. The wheel is rotated slowly
on its axis such that a given portion of the wheel is sequentially
exposed to the two sections. In the first section, the desiccant
is contacted by the process air, or the air to be cooled and dehumidified.
In this section, the desiccant dehumidifies the process air by adsorbing
moisture from this air.
In the second section of the desiccant wheel, the desiccant is
contacted with the regeneration air. The regeneration air evaporates
the moisture from the desiccant that the desiccant adsorbed from
the process air, thereby regenerating the desiccant. By the wheel
rotating through these two air streams, the adsorbing/desorbing
operation of the wheel is continuous and occurs simultaneously.
Generally, the typical system, as shown in Prior Art FIG. 4 operates
by passing the air to be conditioned, or process air, through the
dehumidification section of the desiccant wheel wherein the air
is dehumidified and warmed. This warming occurs from the latent
heat of the water adsorbed onto the desiccant and from the heat
of adsorption generated by this process. Upon exiting the desiccant
wheel, the process air passes through one side of an air-to-air
heat exchanger. In this heat exchanger, the process air gives up
some of the heat it picked up in the desiccant wheel to the air
stream which is to be used to regenerate the desiccant wheel. After
passing through the air-to-air heat exchanger, the process air is
cooled by passing it through the dry side of an indirect evaporative
cooler and then is humidified and further cooled by passing it through
a direct evaporative cooler. The cool, moist air exiting the direct
evaporative cooler is then supplied to the space to be conditioned.
Part of the air leaving the space to be cooled is exhausted and
makes up a portion of the regenerative air stream. The remaining
exhaust air is recirculated and mixed with ambient air to make up
the process air.
The desiccant used to dehumidify the process air must be periodically
regenerated in order for it to remain effective at drying the process
air. This regeneration is accomplished by passing warm or hot air
through the wheel in order to evaporate the water from the desiccant
into the air stream. In the typical system, this warm or hot air
is made up of ambient air which is first passed through the air-to-air
heat exchanger where it picks up some of the heat from the process
air. The regenerative air stream is then passed through a heating
apparatus to further heat the air before it enters the desiccant
wheel. After heating, the regeneration air stream is passed through
the regenerative section of the desiccant wheel in which it evaporates
moisture from the wheel. The regenerative air stream is exhausted
after it passes through the desiccant wheel.
Two general problems are associated with the typical desiccant
based air conditioning systems. First, the air-to-air heat exchanger,
which is used to transfer the heat energy from the dried process
air leaving the desiccant wheel to the regeneration air stream,
is costly. This drives up the first cost of the desiccant based
air conditioning systems thereby limiting their application. In
addition, the amount of heat recovered from the process air and
transferred to the regeneration air stream typically only accounts
for 30-35% of the total heat energy required for this regeneration
air stream. Accordingly, the second problem associated with the
typical desiccant system is that these systems require a significant
amount of energy to sufficiently heat the regeneration air stream
to allow it to effectively dry the desiccant. In some applications
where there is a local supply of inexpensive fuel or if there is
a supply of waste heat, this is not a problem. However, in the vast
majority of applications this will be a significant disadvantage
to the use of desiccant based air conditioning systems. A system
which required less energy to regenerate the desiccant wheel would
reduce the operating cost of desiccant based air conditioning systems
thereby making them cost effective in a greater number of applications.
SUMMARY OF THE INVENTION
The present invention provides a desiccant based air conditioning
system that does not require the expensive air-to-air heat exchanger
which is common to conventional desiccant based systems. In addition,
the desiccant based air conditioning system of the present invention
requires significantly less energy to regenerate the desiccant than
typical systems. In general, these features are accomplished by
utilizing two different adsorbing means in order to make full use
of the latent heat of the ambient air during the regeneration process.
The system of the present invention comprises two different adsorbing
materials which could be contained in beds or on rotating desiccant
wheels. In addition, the system comprises an indirect evaporative
cooler whereby the process air can be cooled by passing through
the dry side of the cooler. A direct evaporative cooler is also
a part of the system. This direct evaporative cooler cools and humidifies
the air being conditioned prior to the air being supplied to the
space to be conditioned. The system also comprises a means of heating
the air which is used for regenerating the desiccant which, in turn,
is used to dehumidify the process air. This means of heating could
be gas-fired, electric, or steam. However, the amount of heat that
must be added in the present invention will be significantly less
than the amount required in typical desiccant based air conditioning
systems. Finally, the present system must also comprise a ducting
means to transport the air streams to the various components of
the present invention.
The present invention includes three basic air streams: a process
air stream and two regeneration air streams. As mentioned previously,
the typical desiccant based air conditioning system will only have
two air streams comprising a process air stream and a single regeneration
air stream. The process air stream in the present invention first
is dehumidified and warmed by passing through the first adsorbing,
or desiccant apparatus. This air stream is then cooled by passing
through the dry side of the indirect evaporative cooler and then
is humidified and further cooled by passing through the wet side
of the direct evaporative cooler. Upon exiting the direct evaporative
cooler, the process air is fully conditioned and is supplied to
the space to be cooled and conditioned.
The first regeneration air stream is used to regenerate the desiccant
that was used to dehumidify the process air stream. This air stream
comprises ambient air that is first passed through the wet side
of the indirect evaporative cooler wherein it becomes almost completely
saturated with moisture and warmed due to the heat given up by the
process air on the dry side of the indirect evaporative cooler.
Upon exiting the wet side of the indirect evaporative cooler, the
first regeneration air stream is contacted with a second desiccant
means whereby the first regeneration air stream is dehumidified.
This second desiccant means will typically operate at a higher moisture
content than the first desiccant means. Because the first regeneration
air stream was almost completely saturated with moisture when it
entered the second desiccant means, the temperature of this air
stream after leaving the desiccant after dehumidification will be
substantially increased due to the latent heat of vaporization and
the heat of adsorption that is generated during the adsorbing process
and is transferred to the first regeneration air stream. As a result,
when the first regeneration air stream leaves the second desiccant
means and enters the heating means, the amount of heat that must
be added to the air is substantially less than what would otherwise
have to have been added in a typical system. Upon exiting the heating
means, the regeneration air stream is brought in contact with the
first desiccant means. When this occurs, the regeneration air stream
evaporates the moisture from the first desiccant that was adsorbed
in the process of dehumidifying the process air. The first regeneration
air stream is exhausted upon exiting the first desiccant means.
The second regeneration air stream is used to regenerate the second
desiccant that was used to dehumidify the first regeneration air
stream. The second regeneration air stream is made up entirely of
ambient air. Because the second desiccant operates at a high moisture
content, the air to regenerate this desiccant need not be as hot
or dry as is typically required to regenerate desiccants which operate
at lower moisture contents. In fact, ambient air is usually sufficient
to evaporate the moisture from the second desiccant that was adsorbed
from the first regeneration air stream. Although heating the ambient
air used to regenerate this second desiccant is usually not necessary,
in some cases where the ambient air is cold or humid, such as during
the winter time, some heating may be required.
The present invention improves upon the typical desiccant based
air conditioning system in several important ways. First, the need
for the air-to-air heat exchanger is eliminated in the present invention.
Instead, the heat of the process air leaving the first desiccant
is transferred to the regeneration air stream in the indirect evaporative
cooler. The elimination of this heat exchanger will lower the first
cost of the present desiccant based air conditioning system.
In addition, the present invention requires substantially less
regeneration energy than typical desiccant based air conditioning
systems. As a result, the size of the heating apparatus will be
reduced, but more importantly, the cost to operate the present invention
will be significantly less. This lower operating cost will likely
allow the desiccant system of the present invention to be cost effective
in many cases where typical desiccant systems were not.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings,
FIG. 1 is a schematic of a desiccant based air conditioning system
utilizing desiccant wheels in accordance with the present invention;
FIG. 2 is a psychrometric diagram showing the path of the process
air stream of the present invention;
FIG. 3 is a psychrometric diagram showing the path of the first
regeneration air stream in the desiccant system of the present invention;
FIG. 4 is a Prior Art Figure showing a schematic of a typical desiccant
based air conditioning system utilizing a single desiccant wheel;
FIG. 5 is a Prior Art Figure of a psychrometric diagram showing
the path of the regeneration air stream in a typical desiccant system;
FIG. 6 is a schematic of a reversible desiccant based air conditioning
system in accordance with the present invention while operating
in the forward mode; and
FIG. 7 is a schematic of a reversible desiccant based air conditioning
system in accordance with the present invention while operating
in the reverse mode.
DETAILED DESCRIPTION
Referring now to FIG. 1 there is shown a schematic of the preferred
embodiment of the desiccant based air conditioning system of the
present invention. In general, it will be seen from this schematic
that there are three main air streams: the process air stream, the
first regeneration air stream, and the second regeneration air stream.
The main components of this system include a first desiccant wheel
42. First desiccant wheel 42 typically comprises a plurality of
desiccant coated substrates which are arranged in a rotating wheel
apparatus. These substrates are generally designed to provide the
greatest possible surface area to maximize the contact area for
the desiccant and the air stream passing therethrough. Common substrate
shapes include a honeycomb arrangement and an arrangement comprising
a plurality of thin plastic sheets of increasing radius of curvature
and arranged concentrically around the axis of the wheel. These
wheels typically range in size from about 3 feet to over 13 feet
in diameter and from about one or two inches to over one foot wide.
A desiccant wheel is generally coupled to an electrically driven
motor which rotates the wheel at speeds ranging from about one or
two revolutions per minute up to about 20 revolutions per minute.
The desiccant affixed to wheel 42 could be selected from a number
of different desiccant materials including silica gel, activated
alumina, molecular sieves, and hygroscopic salts.
Desiccant wheel 42 is placed within an air duct 43 containing a
duct divider 45. Duct divider 45 typically comprises a sheet metal
form which divides the space within the duct in a "V"
notch shape. Duct divider 45 divides duct 43 and hence, desiccant
wheel 42 into two sections. Desiccant wheel 42 is divided such
that section 16 of the wheel will comprise the dehumidification
section of the wheel and will be exposed to the section of duct
43 containing the process air and such that section 17 of the wheel
will comprise the regeneration section of the wheel and will be
exposed to the section of duct 43 containing the regeneration air.
Desiccant wheel 42 rotates about axis 44 such that a given portion
of the wheel first is exposed to the section of duct 43 containing
the process air stream 14 and thus is a part of the dehumidification
section 16 of the wheel. As the wheel rotates, this portion of the
wheel is then exposed to the section of duct 43 containing the regeneration
air stream 34 and thus is a part of the regeneration section 17
of the wheel.
The system of the present invention also includes an indirect evaporative
cooler 46. This cooler has a dry side through which process air
may pass and it has a wet side through which regeneration air may
pass. The air streams flowing through the dry and wet sides of this
cooler do not directly contact each other. Typical indirect evaporative
coolers may be of either the integrated type or the separated type.
In the integrated type of indirect evaporative cooler, a single
heat transfer media is utilized within a single enclosure. This
media is configured such that one side is wetted and allows circulating
water to directly contact an air stream passing therethrough. The
other side of the media is not wetted and the air passing through
does not directly contact any water. In addition, the air streams
on the wetted side and on the dry side are separated by the media
and are not allowed to contact each other. In the separated type
of indirect evaporative cooler, a cooling tower is employed for
the wet air stream. The water from the cooling tower is circulated
through a finned coil which is contained in a separate enclosure
through which the dry air stream would flow.
The outlet of the dehumidification section 16 of desiccant wheel
42 is connected to the dry side inlet 20 of indirect evaporative
cooler 46. The outlet of the dry side of indirect evaporative cooler
46 is connected to the inlet of direct evaporative cooler 48. In
passing through direct evaporative cooler 48 the process air stream
is brought in direct contact with cooling water. Direct evaporative
coolers commonly use a single heat transfer media which allows for
direct contact between the air stream flowing through the cooler
and the water being recirculated in the cooler. Direct evaporative
coolers are commonly referred to evaporative coolers in the industry.
The outlet of direct evaporative cooler 48 is connected to the space
to be conditioned.
As stated previously, indirect evaporative cooler 46 also has a
wet side through which air passes. The wet side inlet 29 of indirect
evaporative cooler 46 is connected to a supply of ambient air 28
which makes up the first regeneration air stream. The wet side outlet
31 of indirect evaporative cooler 46 is connected to a second desiccant
wheel 52.
Second desiccant wheel 52 is of a design similar to that of first
desiccant wheel 42. However, the desiccant contained on second desiccant
wheel 52 will generally have a higher moisture retention capacity
than will the desiccant contained on first desiccant wheel 42. However,
the desiccant affixed to second desiccant wheel 52 could also be
selected from a group of desiccant including silica gel, activated
alumina, molecular sieves, and hygroscopic salts.
Second desiccant wheel 52 is placed within an air duct containing
a duct divider 55. Duct divider 55 is of the same general type as
duct divider 45 previously described. Duct divider 55 divides duct
53 and hence second desiccant wheel 52 into two sections. Section
58 of the wheel will comprise the dehumidification section of the
wheel and will be exposed to the section of duct 53 containing first
regeneration air stream 30. Section 60 of the wheel will comprise
the regeneration section of the wheel and will be exposed to the
section of duct 53 containing second regeneration air stream 38.
Second desiccant wheel 52 rotates about axis 54 such that a given
portion of the wheel first is exposed to the section of duct 53
containing first regeneration air stream 30 and thus is a part of
dehumidification section 58 of the wheel. As the wheel rotates,
this portion of the wheel is then exposed to the section of duct
53 containing second regeneration air stream 38 and thus is a part
of regeneration section 60 of the second desiccant wheel 52.
The outlet of dehumidification section 58 of second desiccant wheel
52 is connected to the inlet of air heating apparatus 56. This heating
apparatus could be of several different conventional types such
as direct gas fired, steam pipe, and electrical resistance. Generally,
however, the heating apparatus of the present invention will be
smaller than the heating apparatus required for typical desiccant
systems because the amount of heat that is required in the present
invention to heat the first regenerative air stream is significantly
reduced. The outlet of air heating device 56 is connected to the
regeneration section 17 of the first desiccant wheel 42.
Still referring to FIG. 1 the operation of the present invention
will now be described. The process air stream is made up of ambient
air 10 and recirculated air 11. Typically, the ambient air stream
will constitute about 25% and the recirculated air about 75% of
this process air stream. These two air streams are mixed at point
12 to form the process air stream 14. Process air stream 14 enters
the dehumidification section 16 of first desiccant wheel 42. While
passing through this section, the desiccant adsorbs moisture from
the process air stream thereby dehumidifying it. As a result of
this process, the temperature of the process air stream is significantly
increased due to the latent heat of the moisture adsorbed and the
heat of adsorption that is generated. The hot, dry air stream 18
leaving first desiccant wheel 42 then enters the dry side of indirect
evaporative cooler 46 through inlet 20 wherein it is cooled. Upon
exiting indirect evaporative cooler 46 at dry side exit 21 the
process air stream passes through direct evaporative cooler 48 wherein
the process air is adiabatically saturated thereby humidifying and
further cooling the process air. Upon exiting direct evaporative
cooler 48 the process air is fully conditioned and can be supplied
to the space to be conditioned 50. Typically, the space to be conditioned
will be an office building, grocery store, or some other area requiring
a supply of cool air. In this space to be conditioned, both heat
and moisture are added to the air stream. Upon exiting this space,
a portion of the process air is exhausted and a portion is recirculated.
Recirculated air stream 11 is mixed with ambient air 10 to comprise
process air stream 14. The amount of the air that is exhausted and
the amount of ambient air 10 which is taken in will be equivalent
in order to maintain a constant air flow rate through the system.
The first regeneration air stream is made up entirely of ambient
air 28. Ambient air 28 first passes through the wet side of indirect
evaporative cooler 46 wherein the air stream is placed in direct
contact with the circulating water of the cooler. By this process,
the ambient air becomes saturated with moisture and picks up the
heat which had been transferred to the circulating water from the
process air stream flowing through the dry side of indirect evaporative
cooler 46. Upon exiting the wet side of cooler 46 at 31 the first
regeneration air stream passes through dehumidification section
58 of second desiccant wheel 52. As stated previously, the desiccant
affixed to this wheel will normally operate at a higher moisture
content than the desiccant affixed to the first desiccant wheel
42.
When the second desiccant is contacted by the warm and humid first
regeneration air stream, the desiccant adsorbs moisture from the
air stream thereby dehumidifying and heating this air stream. Because
first regeneration air stream 30 is almost completely saturated
with moisture when it enters the second desiccant wheel 52 and because
the desiccant affixed to second desiccant wheel 52 operates at a
high moisture content, second desiccant wheel 52 adsorbs a substantial
amount of moisture from first regeneration air stream 30. Since
the amount of heat generated by the adsorption process is directly
related to the amount of moisture that is adsorbed, the amount of
heat given off by this process and transferred to first regeneration
air stream 30 is also substantial. As a result, the temperature
of first regeneration air stream 30 upon leaving second desiccant
wheel 52 at 32 will be significantly increased by this process.
Upon leaving dehumidification section 58 the first regeneration
air stream now at 32 then passes through heating means 56 wherein
additional heat is added to the air stream to effect an additional
increase in temperature of this air stream. However, since the temperature
of first regeneration air stream 30 had previously been significantly
increased as exiting stream 32 due to the adsorption process of
second desiccant wheel 52 the amount of heat that must be added
by heater 56 is much less than would otherwise need to be added
in a typical system.
After passing through heater 56 the hot, dry first regeneration
air stream now at 34 is passed through regeneration section 17 of
the first desiccant wheel. When the desiccant in this section is
contacted by hot and dry first regeneration air stream 34 the moisture
that the desiccant had adsorbed from the process air stream is evaporated
from the desiccant and is carried away. First regeneration air stream
is exhausted as exhaust stream 36 upon leaving dehumidification
section 17 of first desiccant wheel 42.
Second regeneration air stream 38 is also made up of ambient air.
This air stream is passed through regeneration section 60 of second
desiccant wheel 52. Since the desiccant affixed to second desiccant
wheel 52 operates at a high moisture content, the air needed to
regenerate the desiccant affixed to second desiccant 52 need not
be at the high temperatures commonly required for typical regeneration
processes. As a result, the ambient air will, in most cases, evaporate
the moisture from second desiccant wheel 52 when it is brought in
contact with the desiccant. There may, however, be some instances
where a minimal amount of heat will need to be added to the ambient
air to enable it to fulfill its regeneration function. Typically,
this could occur in cases where the ambient air is cold or humid,
such as during the winter months.
FIG. 2 is a Psychrometric Diagram showing the conditions of the
process air at the various stages of conditioning by the present
invention. A psychrometric diagram shows the thermodynamic properties
and relationships of moist air. This diagram has Moisture Content
as its ordinate, Dry Bulb Temperature as its abscissa, and has both
Enthalpy and Saturation Temperature scales bordering its upper left
hand corner.
The reference numerals shown on FIG. 2 directly correlate to the
reference numerals used to describe the system of FIG. 1. As a result,
the condition of the process air at each stage of the system of
the present invention shown by FIG. 1 can be determined from referencing
FIG. 2.
Referring now to FIG. 2 the conditions of the process air at each
step of its conditioning by the present invention will be explained.
Process air 14 is comprised of ambient air 10 and recirculated air
11. The process air 14 first is contacted with the first desiccant
which adsorbs moisture from the process air. As a result of this
process, the process air is dehumidified and the dry bulb temperature
of the air is increased due to the latent heat of vaporization and
the heat of adsorption that is generated. When the process air leaves
the first desiccant, it is at condition 18 shown on the psychrometric
diagram. The process air is then passed through the dry side of
the indirect evaporative cooler in which the process air is cooled
but no moisture is added to the air stream. This operation is shown
as a horizontal line, or constant moisture content, between points
18 and 22 on the psychrometric diagram given in FIG. 2. The process
air leaves the indirect evaporative cooler at condition 22 and enters
the direct evaporative cooler wherein the process air is adiabatically
saturated to condition 24. The process air at condition 24 is fully
conditioned and can be supplied to the space requiring cooling.
In the space to be cooled, both heat and moisture are added to the
process air. Upon leaving the space to be cooled, the process air
is at condition 11. A certain portion of this air is recirculated
and mixed with ambient air 10 to maintain a steady flow of process
air 14 through the system.
FIG. 3 is a Psychrometric Diagram showing the conditions of the
first regeneration air stream at the various stages of the present
invention. Again, the reference numerals used for FIG. 3 directly
correspond to the reference numerals shown for the present invention
in FIG. 1. As a result, the condition of the first regeneration
air stream at each stage of the system of the present invention
shown by FIG. 1 can be determined from referencing FIG. 3.
Referring now to FIG. 3 the conditions of the first regeneration
air stream will be explained. The first regeneration air stream
is comprised entirely of ambient air 28. This air stream is first
passed through the wet side of the indirect evaporative cooler and
is therefore, placed in direct contact with the recirculating water
of this cooler. By this process, the first regeneration air stream
becomes almost completely saturated with moisture and absorbs the
heat from the recirculated water that was transferred from the process.
The exact path that the first regeneration air stream follows during
this process depends upon the various operating conditions of the
system. The path shown on FIG. 3 between point 28 and point 30 is
representative of this process. As can be seen from this chart,
the air at condition 30 has both greater moisture content and greater
enthalpy, or heat, than did the air entering the indirect evaporative
cooler at condition 28.
Upon exiting the wet side of the indirect evaporative cooler at
condition 30 the first regeneration air stream enters the dehumidification
section of the second desiccant wheel. In contacting the desiccant,
the desiccant adsorbs moisture from the first regeneration air stream
which dehumidifies the air and increases its temperature due to
the latent heat and heat of adsorption generated by this drying
process. The first regeneration air stream exits the second desiccant
wheel at condition 32 a condition of lower moisture content but
increased dry bulb temperature, and enters the heating device. While
passing through this device, the temperature of the first regeneration
air stream is increased further while the moisture content of the
air remains constant. The first regeneration air stream exits the
heater at condition 34 which is the condition required to regenerate
the first desiccant.
The amount of heat that must be added to the first regeneration
air stream in the present invention can be seen by referring to
FIG. 3. The total regeneration energy required to condition the
first regeneration air stream from condition 28 to condition 34
is shown as "A" on this diagram. However, in the present
invention, this entire amount of heat does not need to be added
by the heating means. Rather, the first regeneration air stream
is heated from condition 28 to condition 32 by humidifying and heating
the first regeneration air stream on the wet side of the indirect
evaporative cooler and then by drying this air stream in second
desiccant wheel. As a result, the present invention only requires
that sufficient external energy be added in the heating means to
raise the temperature of the first regeneration air stream from
condition 32 to condition 34 which is shown on the diagram as "B".
Prior Art FIG. 4 is a schematic diagram of a typical desiccant
based air conditioning system. As described previously, the typical
system will comprise a desiccant wheel 86 an air-to-air heat exchanger
88 an indirect evaporative cooler 90 a direct evaporative cooler
92 and a heating device 78. Process air 84 is typically comprised
of ambient air 80 and recirculated air 82. Process air 84 first
passes through desiccant wheel 86 whereby process air 84 is dehumidified
and heated. Upon leaving desiccant wheel 86 process air at 87 passes
through air-to-air heat exchanger 88 wherein a portion of the heat
from the process air is transferred to regeneration air stream 70.
Upon exiting air-to-air heat exchanger 88 process air at 89 is
cooled by passing through indirect evaporative cooler 90 and then
is cooled further and humidified by passing through direct evaporative
cooler 92. Upon leaving direct evaporative cooler 92 process air
at 93 is fully conditioned and is supplied to the space to be cooled
94. In the space to be cooled, process air increases in heat and
moisture content and exits at 95. A portion of process air is exhausted
at 96 and the remaining process air is recirculated as stream 82.
Still referring to Prior Art FIG. 4 regeneration air stream 70
is comprised of ambient air which is first passed through air-to-air
heat exchanger 88 wherein it picks up heat from the process air
stream. Upon exiting air-to-air heat exchanger 88 regeneration
air stream now at 72 is heated further by passing through heating
device 78. Upon exiting heating device 78 regeneration air stream
74 is capable of performing its intended regeneration function and
is passed through desiccant wheel 86 wherein it evaporates moisture
from desiccant wheel 86. Upon exiting desiccant wheel 86 regeneration
air stream 76 is exhausted.
The amount of external heat energy that must be added to the regeneration
air stream by heating device 78 in a typical desiccant based air
conditioning system can be seen from FIG. 5. Shown on FIG. 5 is
a psychrometric diagram on which the path of the conditions of the
typical system regeneration air stream is plotted. The reference
numerals utilized on FIG. 5 directly correspond to the reference
numerals which were used to describe the regeneration air stream
of the typical system shown on FIG. 4. As a result, the condition
of the typical system regeneration air stream at each stage of the
typical system shown by FIG. 4 can be determined from FIG. 5.
Referring to FIG. 5 and as described above, the regeneration air
stream in a typical system is made up of ambient air 70. This air
is first passed through an air-to-air heat exchanger in which it
picks up heat from the process air which is shown as path 71. Upon
exiting the air-to-air heat exchanger at condition 72 the regeneration
air must then pass through a heating device, shown as path 73. The
regeneration air stream exits the heating device at condition 74
at which it is fully capable to act as regeneration air.
The total amount of regeneration energy that must be added in the
typical system is equal to the difference in enthalpy between conditions
70 and 74. This difference is shown as "C" on FIG. 5.
The amount of external heat that must be added to the regeneration
air stream by the heating means in the typical system is equal to
the difference in enthalpy of the air between condition 72 and condition
74. This amount of heat is shown on the diagram as "D".
As shown by FIG. 5 the amount of external heat that must be added
by the heating device in the typical system, shown as "D"
on FIG. 5 constitutes approximately 70% of the total heat required,
"C". This is substantially more than the amount of external
heat that is required in the present invention. In fact, referring
back to FIG. 3 it is shown that the amount of external heat that
must be added by the heating device in the system of the present
invention, shown as "B", constitutes only about 50% of
the total heat required in the present invention, shown as "A".
This reduction in the amount of external heat required will significantly
reduce the operating costs of the system of the present invention
when compared to the typical system, thereby making the system of
the present invention more cost effective in a greater number of
applications.
Whereas in the primary embodiment the two desiccants were affixed
to desiccant wheels and the processes of adsorption and desorption
occurred simultaneously and continuously on each wheel, it is also
possible to configure the present invention in such a manner that
the desiccant wheels of the primary embodiment are not required.
To do this, it is necessary to configure the system such that the
flow streams can be periodically reversed to allow regeneration
of the desiccants. One example of such a configuration is shown
by FIG. 6 and FIG. 7. The system shown on these figures does not
require the desiccant wheels of the primary embodiment; but, rather,
the desiccant is contained in beds through which air may pass and
contact the desiccant. FIG. 6 is a schematic diagram of this reversible
system for the forward mode of operation. FIG. 7 is a schematic
diagram of this reversible system for the reverse mode of operation.
As with the primary embodiment described previously, there are
three flow streams in the alternative embodiment of the present
invention shown on FIGS. 6 and 7. These streams include: a process
air stream, a first regeneration air stream, and a second regeneration
air stream. It should be noted that the steps by which each of these
air streams are conditioned are similar to the same as in the primary
embodiment. The differences between the primary embodiment and this
reversible embodiment relate to the equipment utilized and the fact
that the alternative embodiment must be periodically reversed.
There are four desiccant devices in this alternative embodiment.
The first and third desiccant devices will contain a desiccant of
normal moisture retention capacity and are used to dehumidify the
process air stream. The second and fourth desiccant devices will
generally contain a desiccant of high moisture retention capacity
and are used to dehumidify the first regeneration air stream. At
all times during the operation of this system, two of the four desiccant
devices will be operating to dehumidify an air stream while the
other two devices will be being regenerated.
Referring now to FIG. 6 the operation of the alternative embodiment
of the present invention in the forward mode will now be explained.
The solid lines shown on this system schematic represent flow paths
which are used in the forward mode of operation of this system.
The dotted lines represent flow paths which are not used in the
forward mode operation but will be used in the reverse mode. In
the forward mode, first desiccant device 106 is operating to dehumidify
process air stream at 104 second desiccant device 144 is being
used to dehumidify first regeneration air stream at 147 third desiccant
device 156 is being regenerated by first regeneration air stream
at 154 and fourth desiccant device 170 is being regenerated by
second regeneration air stream at 167.
In the forward mode, the process air stream is made up of ambient
air 100 and recirculated air 132. In the forward mode of operation,
valve 135 is open to allow recirculated air 132 to mix with ambient
air 100 at point 102 to form process air 104. The process air stream
is then passed over first desiccant apparatus 106 which will most
likely consist of a desiccant bed. A desiccant bed typically comprises
a column which is filled with loose, spherical shaped desiccant
beads. The bottom of the column is usually porous to allow air to
pass vertically upward through the desiccant. Since the desiccant
beads are mostly spherical in shape, passageways around the desiccant
for air flow-through are created. Generally, the desiccant beads
range in size from about 3 millimeters to 9 millimeters in diameter.
The desiccant columns typically are in the size range of about 5
inches to several feet in diameter. In other instances, the desiccant
contained in these beds is not in the form of loose beads but, rather,
is deposited on a substrate. The substrate will be such as to maximize
the surface area of the desiccant to allow for maximum contact with
the air stream passing therethrough.
When the process air is passed over the desiccant contained in
first desiccant apparatus 106 the desiccant adsorbs moisture from
the process air stream thereby dehumidifying and warming this air
stream. Upon leaving first desiccant apparatus 106 process air
at 108 passes through switching means 110 which directs the process
air to dry side inlet 114 of indirect evaporative cooler 116. Switching
means 110 is typically a damper located within the air duct system.
The damper is generally actuated by an electric motor in response
to control signals, but could also be pneumatically controlled.
In passing through indirect evaporative cooler 116 the process
air stream is cooled while the moisture content of the air remains
constant. Cooled process air passes through the indirect evaporative
cooler dry side exit 118 and passes through direct evaporative cooler
122 wherein the process air is adiabatically saturated with moisture;
thereby, humidifying and further cooling the process air. Upon exiting
direct evaporative cooler 122 air at 124 is fully conditioned and
can be supplied to the space to be conditioned 126. Upon leaving
this space, a certain amount of the process air is exhausted at
130 and the remainder is recirculated through valve 135 to be mixed
with ambient air 100 to comprise process air to be conditioned 104.
First regeneration air stream 134 is comprised entirely of ambient
air. First regeneration air stream 134 first passes through wet
side 136 of indirect evaporative cooler 116 in which it becomes
almost completely saturated with moisture and picks up the heat
that had been transferred from the process air stream to the recirculated
water in this cooler. Upon passing through wet side exit 138 of
indirect evaporative cooler, first regeneration air stream at 140
passes through second switching device 142 which directs first regeneration
air stream now at 147 to second desiccant device 144. The desiccant
contained in second desiccant device 144 will typically operate
at a higher moisture content than the desiccant used in first desiccant
device 106. The desiccant in second desiccant device 144 adsorbs
a significant amount of moisture from the almost saturated first
regeneration air stream and, as a result, effects a substantial
increase in the temperature of this air stream as it passes over
the desiccant. After leaving second desiccant device 144 first
regeneration air stream at 145 then passes through third switching
means 146 which directs first regeneration air stream through air
moving device 148 and through heater 150. Air moving device 148
is typically a fan or blower which is powered by an electrical motor.
The fan could be either of a centrifugal, or "squirrel cage"
type, or could be of the axial fan type.
In passing through the heater 150 the temperature of the first
regeneration air stream is further increased. However, as in the
primary embodiment, the amount of heat that must be added in this
embodiment is significantly less than that required for typical
systems. Upon leaving heater 150 the hot, dry first regeneration
air stream at 153 passes through first switching device 110 which
directs hot, dry first regeneration air stream now at 154 to third
desiccant apparatus 156. As stated previously, the desiccant contained
in third desiccant apparatus 156 is being regenerated by first regeneration
air stream 154 in the forward mode of operation. As a result, first
regeneration air stream 154 evaporates and carries away moisture
from the desiccant in third desiccant device 156 that this desiccant
had adsorbed from the process air when the system was operating
in the reverse mode. First regeneration air stream is exhausted
at 158 upon leaving third desiccant device 156.
Second regeneration air stream 166 in the forward mode is also
comprised entirely of ambient air. Second regeneration air stream
166 passes through third flow switching device 146 which directs
second regeneration air stream to the fourth desiccant device 170.
The desiccant of this fourth apparatus is also generally a desiccant
with a higher moisture content than the desiccant contained in the
first and third desiccant devices, 106 and 156 respectively. In
the forward mode, the desiccant of fourth desiccant device 170 is
in the process of being regenerated by the second regeneration air
stream 166. Since the desiccant of fourth desiccant device 170 operates
at a high moisture content, it can be generated in most cases by
unheated ambient air. After passing through fourth desiccant device
170 second regeneration air stream 171 passes through fourth flow
switching device 172 and is exhausted.
Referring now to FIG. 7 the operation of the alternative embodiment
of the present invention in the reverse mode will now be explained.
The solid lines shown on this system schematic represent flow paths
which are used in the reverse mode of operation of this system.
The dotted lines represent flow paths which were used in the forward
mode operation. For the purpose of clarity, the reference numerals
used on this schematic directly correspond to those used on the
schematic of FIG. 6.
In the reverse mode, first desiccant device 106 is regenerated
by first regeneration air stream at 108 second desiccant device
144 is regenerated by second regeneration air stream at 145 third
desiccant device 156 is used to dehumidify process air at 158 and
fourth desiccant device 170 is being used to dehumidify first regeneration
air stream at 171.
In the reverse mode of operation, valve 164 is open to allow recirculated
air 165 to mix with ambient air 162 at point 160 to form process
air 158. Process air stream 158 is then passed over third desiccant
device 156 which will most likely consist of a desiccant bed. When
the process air stream 158 passes over the desiccant contained in
third desiccant device 156 the desiccant adsorbs moisture from
the process air stream thereby dehumidifying and warming this air
stream. Upon leaving third desiccant apparatus 156 process air
154 passes through switching means 110 then through the dry side
of indirect evaporative cooler 116 and then through direct evaporative
cooler 122. Upon exiting direct evaporative cooler 122 the process
air at 124 is fully conditioned and can be supplied to the space
to be conditioned 126. Upon leaving this space, a certain amount
of the process air is exhausted at 130 and the rest at 165 is recirculated
to be mixed with ambient air 162.
First regeneration air stream 134 in the reverse mode is comprised
entirely of ambient air. First regeneration air stream 134 first
passes through the wet side at 136 of indirect evaporative cooler
116 and then through fourth switching device 172 which directs
the air stream to fourth desiccant device 170 wherein this first
regeneration air stream is dehumidified and warmed. After leaving
fourth desiccant device 170 first regeneration air stream at 167
passes through third switching means 146 which directs first regeneration
air stream through air moving means 148 and through heater 150.
Upon leaving heater 150 hot and dry first regeneration air stream
at 153 passes through first switching device 110 which directs first
regeneration air stream to first desiccant apparatus 106. In the
reverse mode, the desiccant contained in first desiccant apparatus
106 is being regenerated by first regeneration air stream at 108.
First regeneration air stream is exhausted at 100 upon leaving first
desiccant device 106.
Second regeneration air stream 166 in the reverse mode is also
comprised entirely of ambient air. Second regeneration air stream
166 passes through third flow switching device 146 which directs
second regeneration air stream now at 145 to second desiccant device
144. In the reverse mode, the desiccant of second desiccant device
144 is in the process of being regenerated by second regeneration
air stream 145. After passing through second desiccant device 144
second regeneration air stream at 147 passes through second flow
switching device 142 and is exhausted.
The foregoing description has been given to clearly define and
completely describe the preferred embodiment and one alternative
embodiment of the present invention. Various modifications may be
made without departing from the scope and spirit of the invention
which is defined in the following claims. |